Normal human cells are hardly immortalized and age. The normal human cells are destined to be aging according to the mechanism of counting the times of cell division (senescence mechanism). Ordinarily, the a living tissue gradually lose their proliferation capability and display aged morphology after having repeated subculturing (or division) 20 to 80 times. When a cell ages, it shows several morphological and biochemical changes, including cytomegalic tendency, squamous tendency, changes in extracellular matrix components, non-response to stimulation by mitogens and a decline in the expression function of proliferation regulating genes and it ceases to divide; therefore, it will be easy to be so judged.
However, when the cells are subjected to treatment with a carcinogen, or treatment by radiation during subculturing, a very small portion of the cells escape from this cell senescence and continue to grow, thereby forming a colony. The cells that have thus acquired the infinite proliferation capability (or that has experienced the failure of senescence mechanism) can be continuously cultured and do not die even after the passage of a finite number of cell generations: they are referred to as “immortal cells.”
When a normal cell and an immortal cell are fused, the resulting hybrid cell displays finite division capability. Further, when immortal cells are fused with each other, there is obtained a hybrid cell displaying the finite division capability. This suggests that the immortalization is genetically recessive to senescence in human cells and that the deficiency of a particular gene (immortalization-suppressing gene) which is involved in the mechanism of counting the times of cell division (senescence mechanism) or the loss of its function is needed for cell immortalization.
Although it is not easy to directly prepare cancer cells from normal human cells, there is a close relation between the immortalization of cells and the cancerization of cells. It is beginning to be understood that in an experimental system where cells undergo cancerization under culturing conditions the normal human cells evade the senescence mechanism which strongly acts on their proliferation in a negative manner and mutate in a multistage such that the cells change into the immortalization stage allowing infinite proliferation and then into the cancerization stage with relative ease. For example, when the so-called oncogenes such as p53 mutant gene and Rb mutant gene are expressed in normal human cells, neither cancerization nor immortalization occurs; however, once the cells have been immortalized, they will easily turn cancerous by the oncogenes (Namba, M. et al., Crit. Rev. Oncogen., 7:19-31, 1996). This strongly suggests that the immortalization of cells is an important step in the cancerization of cells. Therefore, an analysis of the immortalization stage is critical to elucidating the carcinogenesis of human cells.
For proliferation inhibitory genes in cancer cells, there are mentioned more than ten kinds of genes, including Rb gene in retinoblastoma, p53 gene in colon cancer, and WT gene in Wilms tumor. Particularly, a preferred proliferation inhibitory gene is p53 and gene therapy using p53 has already begun (Li, H. et al., Clinical Cancer Res., 5, 637-642, 1999). Since immortalization of the cells is involved in the cancerization, not only cancer-suppressing genes but also the cell proliferation inhibition by the immortalization-suppressing genes will be effective treatment of cancer.
Thus far, there have been reported as genes associated with cell senescence and immortalization, Sdil (Noda, A. et al., Exp. Cell. Res., 211: 90-98, 1994), SEN6 (Banga, S. S. et al., Oncogene, 14: 313-321, 1997), SEN6A (Sandhu, A. K. et al., Oncogene, 12: 247-252, 1996), ING1 (Garkavtsev, I. and Riabowol, K., Mol. Cell. Biol., 17: 2014-2019, 1997), Hic-5 (Shibanuma, M. et al., Mol. Cell. Biol., 17: 1224-1235, 1997), among others. However, the correlation between these genes and the aging, immortalization, or cancerization of human cells has not been elucidated.